Aromatic polyimides can be produced by condensation polymerization between aromatic diamine compounds and aromatic tetracarboxylic acid compounds followed by curing (imidization). Because of their high mechanical strength, heat resistance, electrical insulating properties, and chemical resistance, various aromatic polyimides are used for electronic substrate materials. With a recent increase in frequency in electronic equipment associated with high-speed signal transmission, there has been a growing demand for polyimides having lower dielectric constants and lower dielectric loss tangents as electronic substrate materials. In electronic circuits, the signal transmission speed decreases with the increasing dielectric constant of substrate materials, and the signal transmission loss increases with increasing dielectric constant and dielectric loss tangent. Thus, lower dielectric constants and dielectric loss tangents of polyimides used as substrate materials are essential for higher performance of electronic equipment. Particularly in communication equipment used at high frequencies, there is a demand for lower dielectric loss tangents.
Examples of polyimides currently used as electronic substrate materials include two-component polyimides, such as p-phenylenediamine (PDA)-3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (sBPDA) polyimides (Patent Literature 1), 4,4′-diaminodiphenyl ether (ODA)-pyromellitic acid dianhydride (PMDA) polyimides (Patent Literature 2), and three- and four-component polyimides produced by copolymerization of these monomers at any ratio (Patent Literature 3). These polyimides have relatively high dielectric constants and dielectric loss tangents due to their polar imide groups.
In order to decrease the relatively high dielectric constants and dielectric loss tangents of these polyimides due to polar imide groups, for example, in a method proposed in Patent Literature 4, the dielectric constant of a polyimide is lowered by introducing a monomer having a long-chain skeleton to decrease the number of imide groups per unit molecular length (the concentration of imide groups) and thereby lower the polarity of the entire molecule. Also in a method proposed in Patent Literature 5, the dielectric constant of a polyimide is lowered by introducing fluorine and an aliphatic ring structure. However, in the former method, many aliphatic chain structures in a polyimide impair the intrinsic characteristics of the polyimide, such as mechanical strength, heat resistance, chemical resistance, and insulating properties. In the latter method, introduction of fluorine causes corrosion and damage due to elimination of fluorine during a heating step, and introduction of an aliphatic ring structure disadvantageously lowers heat resistance compared with aromatic ring structures.
Patent Literature 6 discloses a heat-resistant adhesive that contains a polymer (poly(amic acid)) produced by condensation polymerization between 5-(4-aminophenoxy)-3-[4-(4-aminophenoxy)phenyl]-1,1,3-trimethylindan and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride. However, Patent Literature 6 does not describe the dielectric constant and dielectric loss tangent of a polyimide produced by imidization of the poly(amic acid) and does not inform whether the polyimide is useful for electronic substrate materials, particularly for high-frequency substrate materials.